1. Field of the Invention
The invention relates to a method for operating a device for conveying a fluid. The device can be used, for example, in a motor vehicle in order to convey a liquid additive to an exhaust gas treatment device for purifying the exhaust gases of an internal combustion engine of the motor vehicle, and to feed the liquid additive to the exhaust gas treatment device.
2. Related Art
In exhaust gas treatment devices to which a liquid additive for purifying exhaust gases is fed it is possible, for example, for the method of selective catalytic reduction (SCR method) to be carried out. In this method nitrogen oxide compounds in the exhaust gas of an internal combustion engine are reduced using a reducing agent. The reducing agent that is used is usually ammonia. In motor vehicles, ammonia is usually not stored directly but rather in the form of a reducing agent precursor solution. A frequently used reducing agent precursor solution is liquid urea-water solution. A 32.5 percent urea-water solution can be obtained under the trade name AdBlue®. This reducing agent precursor solution can be fed to an exhaust gas treatment device with a device that can be operated according to the method described below.
During the development, during the manufacture and during the operation of devices for making available liquid additive for purifying exhaust gas, it is generally necessary to take into account the fact that the (aqueous) fluid can freeze at low temperatures. The 32.5 percent urea-water solution freezes, for example, at −11° C. Such low temperatures can occur in the field of motor vehicles, in particular, during relatively long deactivated times in winter. When the fluid freezes, an expansion of volume typically occurs. This expansion of volume can damage the conveying device.
For this reason it is known to empty a device for conveying a fluid when operations stop. When emptying, the fluid is removed from the device and is replaced by air from the surroundings. Typically, air is sucked into the device via an injector on an exhaust gas line, while at the same time the fluid is conveyed back into a tank. Then, during the stationary phase of the motor vehicle there is no fluid present within the device. Correspondingly, no expansion of the volume of the fluid can also occur within the device. It is also not possible for a volume of fluid within the device to expand within the device. However, it is problematic here that an emptied device first has to be filled before it can be re-activated. When the device is refilled, there is, in particular, the risk of leakage occurring. If the device is connected to an exhaust gas treatment device, it is possible for fluid to spill over into the exhaust gas treatment device when refilling is carried out, for example. Furthermore, both when emptying and when filling the device unnecessarily long operation of the pump of the device should be avoided. A short operating period of the pump reduces, on the one hand, the energy consumption during the filling and during the emptying. On the other hand, it is therefore also possible to avoid a situation in which the pump is damaged and/or increased wear of the pump occurs as a result of undesired conveyance of air and/or as a result of the pump working against an increased conveying resistance.